Once Before Time - Martin Bojowald [73]
Data gained from the hills of the anisotropy spectrum thus seem to contradict each other—unless there is another, as yet unknown form of energy that does not lead to clumping; it would be relevant for the spatial curvature and thus the main maximum, but not for smaller hills. The disagreement of interpreted data looks like a severe problem that can be solved only by audaciously postulating a new form of energy, as yet unseen. Worst of all, observations require this energy form to make up about 70 percent of all the energy in the universe! Simply postulating any new form of energy, especially one so dominant, surely requires good reasons; otherwise it could hardly be taken seriously. Cosmology has by now provided a strong set of arguments, based on the fact that the new energy form is required not only by cosmic microwave data but also by other observations discussed in what follows. So the set of all observations is indeed consistent with the postulate of a new form of energy called dark energy, but this is a success paid for by the enigma of the energy’s nature. So far, no satisfactory theoretical explanation has been found despite numerous attempts.
18. Spectrum of the cosmic microwave radiation with a precisely measured main maximum as well as smaller hills. (Image courtesy NASA/WMAP Science Team, http://map.gsfc.nasa.gov/media/080999/index.html.)
Adding insult to injury, the value of all mass not attributed to dark energy is decidedly larger than the amount of matter seen in the form of stars. Following the familiar script, this leads to another postulated energy form: dark matter, as distinguished from dark energy. It is rather prevalent, too, accounting for about 26 percent of the energy in the universe, but unlike dark energy, it does not cause accelerated expansion. For the matter we know, and are made from, only a meager 4 percent remains, a tiny minority. Indications for dark matter existed much longer, inferred from the velocities of stars in galaxies. So it is heartening to see that very different kinds of cosmological observations lead to consistent results, despite the darkness. Dark matter will not be discussed here much because it has few connections to quantum gravity. As dark matter candidates, objects as diverse as black holes and new kinds of elementary particles were proposed; the latter, if they exist, may possibly be found in accelerator experiments.
Another interesting characteristic of background radiation is its polarization. Electromagnetic radiation is of transversal form: Like a rope, it can swing in different spatial directions perpendicular to its own axis. Also the plane in which this swinging occurs, together with its variation over the whole sky, can be measured for cosmic microwave radiation. Recent new measurements by the WMAP satellite have shown that the polarization exists and have indicated the first concrete, though not yet precise, values. New data from the Planck satellite should provide an additional wealth of information about background radiation. Polarization can be caused only by the periodic space-time deformations of gravitational waves, stretching the stage for microwaves to move on, but not by other kinds of space-time curvature as they come from matter. From polarization data one can thus draw conclusions about the intensity of gravitational waves near the big bang, and further test or even constrain theoretical ideas such as traces of quantum gravity.
GALAXY MAPS: MILKY WAYS BY THE MILLIONS
There one begins, a tiny wretch
So happy smallest bites to catch,
Thus one is growing patch by patch
And practices to play a grander match.
GOETHE, Faust
Sometime after the release of the cosmic microwave background, galaxies formed out of those very density variations that are still recognizable by their seeds in the slightly nonuniform background